A plurality of electrochemical cells are connected together to form a battery. Silver-containing material is widely used as a cathode material in electrochemical cells. Silver-containing cathodes typically contain silver carbonate, silver thiocyanate, divalent silver oxide, silver bismuth oxide, copper silver vanadium oxide, and silver vanadium oxide. Some batteries, when using some of these compounds as the cathode material in individual electrochemical cells therein, however, do not exhibit ideal electrical properties. Ideal electrical properties include a low internal discharge rate (i.e., a low increase in internal resistance over the lifetime of the cell). A high internal discharge rate undesirably decreases the deliverable capacity (i.e., the integral of current times the discharge time) of a cell. Different cathode materials contribute to different problems. For example, silver chromate undesirably contributes to a large voltage drop during high loads. Divalent silver oxide is soluble and undesirably decomposes over time. These are just a few of the problems associated with some of the above-mentioned cathode materials.
Silver vanadium oxide (SVO) is utilized as a cathode material in lithium (Li) anode electrochemical cells (and, thus, batteries incorporating such electrochemical cells) due to its relatively high volumetric energy density (i.e., the product of capacity times average voltage divided by volume of material), which is particularly desirable for small batteries. The size of the battery is important in implantable medical devices, such as implantable cardiac defibrillators (such as that illustrated in FIG. 1), so that the device itself occupies a smaller volume within a patient's body and is lighter in weight.
Batteries utilized in implantable medical devices (e.g., defibrillators), however, must also be very reliable. Thus, it is important that such implantable batteries be able to deliver a pulsing current with a minimal voltage drop during the pulse. Typical pulsing requirements include pulses of about ten seconds in duration at an amplitude of about one to about three amperes. Thus, in order to have sufficient available voltage for providing the current pulses, it is important that batteries do not have a high time-dependent increase in internal resistance so that they can provide a long service life without needing frequent replacement.
Because a battery that does not exhibit an increase in internal resistance over the life of the battery is not yet possible, a battery that has a predictable life and provides signs of its chemical exhaustion before the end of its life is important, particularly for use in implantable medical devices. Signs of chemical exhaustion enable one attuned to such signs to change the battery before its failure time. SVO cathodes, when used in conjunction with Li anodes, provide an open circuit voltage curve with multiple voltage plateaus as a function of depth of discharge. Thus, electrochemical cells containing SVO cathodes and Li anodes are preferred for use in implantable batteries because the lifetime of such batteries can readily be detected from the position on and slope of the curve.
SVO is capable of being synthesized using a variety of methods. Methods of synthesis generally fall within two categories, depending on the type of chemical reaction that produces the SVO. SVO can be synthesized using a decomposition reaction, resulting in decomposition-produced SVO (DSVO). Decomposition reactions are known to utilize decomposable metal compounds, such as nitrates, nitrites, carbonates, and ammonium salts for the reacting metal components. A conventional DSVO reaction proceeds at a temperature of about 360.degree. C. from silver nitrate and vanadium pentoxide according to the following reaction: 2AgNO.sub.3 +2V.sub.2 O.sub.5 .fwdarw.Ag.sub.2 V.sub.4 O.sub.11 +2NO.sub.x. This process results in DSVO having a relatively low crystallinity and a capability to have a pressed pellet density of about 3.04 g/cm.sup.3 (pressed under the conditions described herein).
Alternatively, SVO can be synthesized using a combination reaction, resulting in combination-produced SVO (CSVO). A conventional CSVO reaction proceeds at a temperature of about 500.degree. C. from silver oxide and vanadium pentoxide according to the following reaction: Ag.sub.2 O+2V.sub.2 O.sub.5 .fwdarw.Ag.sub.2 V.sub.4 O.sub.11. The resulting CSVO is well crystallized and has a capability to have a pressed pellet density of about 3.53 g/cm.sup.3, which is approximately fifteen-percent greater than the pressed pellet density of DSVO, using the same conditions for pressing the material.
Regardless of how it is made, SVO can be formed in a variety of different structural phases (e.g., .beta., .gamma., and .epsilon.). This is illustrated by the phase diagram for the formation of SVO from the decomposition reaction of V.sub.2 O.sub.5 and AgVO.sub.3 in prior art FIG. 2.
SVO cathode material, when used in conjunction with a Li anode in an electrochemical cell, exhibits varying open circuit voltage characteristics, depending on the amount of Li incorporated into the cathode. As is illustrated in prior art FIG. 3, when used in conjunction with SVO having a formula of Ag.sub.2 V.sub.4l O.sub.11, the amount of Li, x, incorporated into the cathode dictates the open circuit voltage of the electrochemical cell. The graph of open circuit voltage versus the amount of Li, x, incorporated into the cathode illustrates a characteristic dual voltage plateau for such electrochemical cells and batteries. The open circuit voltage is fairly constant at portion 80 of the graph, when x is about 0 to about 2, and at portion 82, when x is about 3 to about 5.2. The open circuit voltages at these two portions 80 and 82 have respective values of about 3.2 V and about 2.6 V. A problem experienced by many LixAg.sub.2 V.sub.4 O.sub.11 electrochemical cells, however, is a time-dependent increase in internal resistance with increasing amounts of lithium, x, in the cathode composition of LixAg.sub.2 V.sub.4 O.sub.11, starting just to the left of the second voltage plateau (i.e., portion 82) of FIG. 3 and continuing the remainder of the discharge time.
To test the behavior of an electrochemical cell containing a particular SVO composition upon discharge over a long period of time, especially the time-dependency starting just prior to the second voltage plateau, long-term discharge tests are run. The results of a long-term discharge test for an electrochemical cell containing a conventional, as-synthesized (i.e., material that has not been processed after its initial formation) DSVO cathode material are illustrated in prior art FIG. 3. In long-term discharge tests, conventional Li/DSVO electrochemical cells display a time-dependent increase in internal resistance when the amount of Li, x, in Li.sub.x Ag.sub.2 V.sub.4 O.sub.11 is about 2.3 to about 6.7. Curve 84 in FIG. 3 corresponds to an accelerated test on the order of magnitude of a few days. Curve 86 corresponds to one more year than curve 84. Curve 88 corresponds to three more years than curve 84. Curve 90 corresponds to five more years than curve 84. The internal resistance values of the four curves increase with increasing time. This increase in internal resistance potentially significantly shortens the lifetime of devices in which it is used.
As stated above, it has been shown that discharge characteristics of a Li/CSVO electrochemical cell are more desirable than the discharge characteristics of a conventional Li/DSVO electrochemical cell. This is particularly due to Li/DSVO's time-dependent increase in internal resistance beginning slightly before the second voltage plateau of a discharge curve. Thus, attempts have been made at improving the electronic properties of DSVO cathode material for use in electrochemical cells and batteries.
For example, attempts have been made to increase the synthesis temperature of DSVO, so that it more closely resembles the higher synthesis temperature of CSVO. U.S. Pat. No. 5,545,497 (Takeuchi et al.) discloses a method of synthesizing DSVO by reacting vanadium pentoxide and silver nitrate at temperatures of 350.degree. C. to 550.degree. C. The resulting DSVO has a formula of Ag.sub.x V.sub.2 O.sub.y, wherein 0.33&lt;x&lt;0.99 and 5.16&lt;y&lt;5.49.
DSVO cathodes with optimal characteristics have been reported to have been synthesized in air at 450.degree. C. R. A. Leising et al., (Chem. of Materials, 5, 738-42 (1993)). This DSVO has the formula AgV.sub.2 O.sub.5.5 and needle-like crystallites. The DSVO crystallites are reported to have a typical crystallite diameter of less than one micron and a length of 10 to 20 microns. However, the capacity (i.e., integral of current times the discharge time) results for electrochemical cells containing DSVO synthesized at 450.degree. C. indicated little difference from capacity results for electrochemical cells containing DSVO synthesized at 320.degree. C. to 375.degree. C., as shown in Table V of Leising et al.
Furthermore, Leising et al. reported that the DSVO synthesized at 450.degree. C. had a similar degree of crystallinity as compared to the DSVO synthesized at 320.degree. C./375.degree. C., utilizing x-ray diffraction (XRD) analysis for that conclusion. Also, Leising et al. reported that when DSVO was synthesized in air at 540.degree. C., the resulting material contained a mixture of different crystallographic phases (e.g., 10.times.40 micron crystallites mixed with irregular particles), and the XRD data indicated the presence of a new phase having a formula of Ag.sub.1.2 V.sub.3 O.sub.8 (-phase)). Testing of this DSVO material in electrochemical cells resulted in an undesirably significant decrease in delivered capacity. Thus, according to Leising et al., increasing the temperature at which the DSVO is synthesized does not improve the electrochemical performance of the DSVO when used as a cathode material.
Thus, there is a need for improved SVO cathode electrochemical cells with decreased time-dependent internal resistance beginning slightly before the second voltage plateau. There is a further need for improved SVO cathode electrochemical cells with increased power and capacity.
Table 1 below lists documents that disclose information of interest to methods of preparation of silver vanadium oxide (SVO) and electrochemical cells containing SVO cathodes, as well as electrochemical cells in general.
TABLE 1 ______________________________________ Patent No. Inventor(s) Issue Date ______________________________________ 4,016,338 Lauck 5 April 1977 4,158,722 Lauck et al. 19 June 1979 4,310,609 Liang et al. 12 Jan. 1982 4,391,729 Liang et al. 5 July 1983 4,542,083 Cava et al. 17 Sept. 1985 4,675,260 Sakurai et al. 23 June 1987 4,751,157 Uchiyama et al. 14 June 1988 4,751,158 Uchiyama et al. 14 June 1988 4,803,137 Miyazaki et al. 7 Feb. 1989 4,830,940 Keister et al. 16 May 1989 4,964,877 Keister et al. 23 Oct. 1990 4,965,151 Takeda et al. 23 Oct. 1990 5,194,342 Bito et al. 16 March 1993 5,221,453 Crespi 22 June 1993 5,298,349 Takeuchi 29 March 1994 5,389,472 Takeuchi et al. 14 Feb. 1995 5,545,497 Takeuchi et al. 13 Aug. 1996 5,458,997 Crespi et al. 17 Oct. 1995 5,472,810 Takeuchi et al. 5 Dec. 1995 5,498,494 Takeuchi et al. 12 March 1996 5,498,495 Takeda et al. 12 March 1996 5,512,214 Koksbang 30 April 1996 5,516,340 Takeuchi et al. 14 May 1996 5,558,680 Takeuchi et al. 24 Sept. 1996 5,567,538 Oltman et al. 22 Oct. 1996 ______________________________________ Leising et al., Chem. of Materials, 5, 738-42 (1993) Zandbergen et al., Journal of Solid State Chemistry, 110, 167-175 (1994)
All documents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments, and Claims set forth below, many of the devices and methods disclosed in the documents in Table 1 may be modified advantageously by using the teachings of the present invention.